I only collect the Buffy figures and I had seen the 6" ones around for quite awhile and they never appealed to me. I just don't think they are very good and then I saw the Hush Buffy on Ebay one day and thought it looked great. I didn't buy it at that time, it wasn't until I saw the GD Buffy, Angel and Spike figures that I got hooked and started collecting them. I just think the sculpts, with a couple of exceptions, are spot on fantastic. The detail in the sculpt and outfits are wonderful.
I think the fact that they are 12" size and look so good makes them that much more special to collect. Like I said, I don't think the 6" ones look that great and there was some other size out for awhile (maybe 10") that looked horrendous and nothing like the actors at all. I also love that they are so posable too.
And, as you can see by my name, I love Firefly/Serenity and would love it if Sideshow did that line too. There are the 6" ones out there but I won't buy those - again, just don't think they look like the characters that much, but if Sideshow started selling those I'd be buying!!